When designing a user interface (UI), a software developer sometimes creates a UI element having a property that is dependent on the property of a second UI element or other source. A change to the property of the second UI element (or other source) causes a change to a property of the first UI element. For example, the size of an image graphic can be programmed to change depending on the size of a window that includes the image graphic. When the size of the window changes, the size of the image graphic changes accordingly. Or, as another example, the sliding scroll indicator in a scroll bar can be programmed to slide to different positions within the scroll bar depending on which part of a list of content is in a display area. When a user scrolls to view a different part of the content, the scroll indicator changes positions.
When programming a property of a UI element to be dependent on a source value, a developer can program software to compute the value of the property as a function of the source value. A code function is one common way to compute the property value. A typical code function accepts the source value as a parameter (or otherwise gets the source value) and returns the value of the property of the first UI element. In some software systems, code functions are simple to implement and run quickly. In other systems, however, code functions can be difficult to implement or slow to run. In particular, in systems in which UI elements are specified in markup language that is processed to render the UI elements, accessing a code function can be difficult or slow. Such systems are common for UI rendering in mobile computing devices and other computing devices.